Multicomponent optical body

ABSTRACT

Multilayer polymeric films and other optical bodies are provided. The films, which have at least three layers of different composition in the optical repeating unit, reflect light in a first portion of the spectrum while transmitting light in a second portion of the spectrum, exhibit improved reflectivities at oblique angles, and can be designed to suppress one or more higher order harmonics of the main reflection band.

This Application is a divisional of U.S. Ser. No. 09/006,118, filed Jan.13, 1998, now U.S. Pat. No. 6,207,260.

FIELD OF THE INVENTION

The present invention relates generally to a multilayer polymeric filmsthat reflect light in a first portion of the spectrum while transmittinglight in a second portion of the spectrum, and in particular to areflective polymeric film having at least three layers of differentcomposition in the optical repeating unit.

BACKGROUND OF THE INVENTION

The use of multilayer films comprising multiple alternating layers oftwo or more polymers to reflect light is known and is described, forexample, in U.S. Pat. No. 3,711,176 (Alfrey, Jr. et al.), U.S. Pat. No.5,103,337 (Schrenk et al.), WO 96/19347, and WO 95/17303. The reflectionand transmission spectra of a particular multilayer film dependsprimarily on the optical thickness of the individual layers. Opticalthickness is defined as the product of the actual thickness of a layerand its refractive index. Accordingly, films can be designed to reflectinfrared, visible or ultraviolet wavelengths λ_(M) of light byappropriate choice of optical thickness of the layers in accordance withthe following formula:

λ_(M)=(2/M)*D _(r)  (Formula I)

wherein M is an integer representing the order of the reflected light,and D_(r) is the optical thickness of an optical repeating unit (alsocalled multilayer stack) comprising two or more polymeric layers.Accordingly, D_(r) is the sum of the optical thicknesses of theindividual polymer layers that make up the optical repeating unit. Byvarying the optical thickness of an optical repeating unit along thethickness of the multilayer film, a multilayer film can be designed thatreflects light over a wide range of wavelengths.

From Formula I, it can also be seen that a multilayer film or opticalbody which is designed to reflect light in a first region of thespectrum may have higher order reflections in a second region of thespectrum. For example, a multilayer film designed to reflect infraredlight will also have higher order reflections in the visible region ofthe spectrum. Specifically, a multilayer film designed to have a firstorder reflection (M=1) at 1500 nm may have higher order reflections atabout 750 nm (M=2), 500 nm (M=3), 375 nm (M=4), etc. A film designed toreflect infrared light of even longer wavelengths may have even morehigher order reflections in the visible region. Thus, for example, amultilayer film having a first order reflection at 2000 nm will havehigher order reflections at 1000 nm, 666 nm, 500 nm, 400 nm, etc. Thesehigher order reflections are undesirable in many applications (e.g.,window films) because they impart an iridescent appearance to the filmwhere a transparent, colorless appearance is preferred. Therefore, inorder to design a multilayer film that reflects light over a firstregion of the spectrum (e.g., the infrared region) but does not reflectlight over a shorter wavelength region (e.g., the visible region), atleast two, and preferably at least three higher order reflections needto be suppressed.

U.S. Pat. No. 5,103,337 (Schrenk et al.) teaches that an infraredreflecting multilayer film having an optical repeating unit withpolymeric layers A, B and C arranged in an order ABC, is capable ofsuppressing at least two successive higher order reflections when theindex of refraction of polymeric layer B is chosen to be intermediate tothat of polymeric layers A and C. In a particular embodiment of the filmdescribed therein, the optical repeating unit is formed by arranginglayers A, B and C in an ABCB pattern. By selecting polymeric materialsA, B and C such that the refractive index of material B equals thesquare root of the product of the refractive index of materials A and C,and by setting the optical thickness ratio for material A and C to ⅓ andthat of material B to ⅙, at least three higher order reflections can besuppressed. Similar teachings are found in Thelen, A., J.Opt.Soc. Am.53, 1266 (1963). However, one disadvantage of this design is that theamount of reflection of incident light with the first order harmonicdecreases with increasing angle of incidence. A further disadvantage ofthis design is that the suppression of the three higher orderreflections also decreases with increasing angle of incidence. Thislater result is particularly undesirable in applications such as windowfilms where the infrared reflective film is used to shield a room frominfrared sunlight, since the sunlight will frequently be incident atangles substantially away from the normal (particularly in the springand summer when the sun is high in the sky).

U.S. Pat. No. 5,540,978 (Schrenk) teaches a multilayer polymeric filmthat reflects ultraviolet light. In one embodiment, the film includesfirst, second, and third diverse polymeric materials arranged in arepeating unit ABCB. In another embodiment, the layers are arranged inthe repeating unit ABC.

WO 96/19346 teaches reflective films that are made out of an opticalrepeating unit of alternating layers A and B, where A is a birefringentpolymeric layer and B can be either isotropic or birefringent. Thereference notes that, by matching the index of refraction between bothlayers along an axis that is perpendicular to the surface of the film,the dependency of reflection on angle of incidence can be greatlyreduced. However, the reference does not teach how these results can beextended to multilayer optical systems having three or more layer typesin the repeating unit (e.g., films with ABC or ABCB repeating units).Such a system would be highly desirably, both because of the improvementin reflectivity at oblique angles it would afford, and because theadditional layer or layers in the repeating unit could be used to impartbetter mechanical properties to the system. For example, one of theadditional layers could be an optical adhesive that would reduce thetendency of the other layers to delaminate. Furthermore, while WO96/19346 mentions infrared reflective films, it does not describe how anIR film can be made that will not suffer from higher order reflectionsin the visible region of the spectrum (e.g., if the first orderreflection is at 1200 nm or more).

There is thus a need in the art for a multilayer film or other opticalbody that exhibits a first order reflection band for at least onepolarization of electromagnetic radiation in a first region of thespectrum (e.g., in the infrared, visible or ultraviolet regions of thespectrum) but can be designed to suppress at least the second, andpreferably also at least the third, higher order harmonics of the firstreflection band. In particular, there is a need in the art for amultilayer film or optical body that has a first reflection band in theinfrared region of the spectrum but that exhibits essentially no higherorder reflection peaks in the visible region of the spectrum.

There is also a need in the art for a film or other optical body havingthree or more layer types in its optical repeating unit, and for whichthe reflectivity of the film (e.g., toward infrared radiation) remainsessentially constant, or increases, at non-normal angles of incidence.

These and other needs are met by the films and optical bodies of thepresent invention, as hereinafter described.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides films and other opticalbodies which exhibit a first order reflection band for at least onepolarization of electromagnetic radiation in a first region of thespectrum, while suppressing at least the second, and preferably also atleast the third, higher order harmonics of the first reflection band.

In another aspect, the present invention provides a multilayer opticalfilm having at least three different layer types in its optical;repeating unit, and for which the % reflection of the first orderharmonic remains essentially constant, or increases, as a function ofangle of incidence. This may be accomplished, for example, by forming atleast a portion of the optical body out of polymeric materials A, B, andC which are arranged in a repeating sequence ABC, wherein A hasrefractive indices n_(x) ^(A), n_(y) ^(A), and n_(z) ^(A) along mutuallyorthogonal axes x, y, and z, respectively, B has refractive indicesn_(x) ^(B), n_(y) ^(B), and n_(z) ^(B) along axes x, y and z,respectively, and C has refractive indices n_(x) ^(C), n_(y) ^(C) andn_(z) ^(C) along axes x, y, and z, respectively, where axis z isorthogonal to the plane of the film or optical body, wherein n_(x)^(A)>n_(x) ^(B)>n_(x) ^(C) or n_(y) ^(A)>n_(y) ^(B)>n_(y) ^(C) andwherein n_(z) ^(C)≧n_(z) ^(B) and/or n_(z) ^(B)≧n_(z) ^(A). Preferably,at least one of the normalized differences 2(n_(z) ^(A)−n_(z)^(B))/(n_(z) ^(A)+n_(z) ^(B)) and 2(n_(z) ^(B)−n_(z) ^(C))/(n_(z)^(B)+n_(z) ^(C)) is less than about −0.03.

Surprisingly, it has been found that, by designing the film or opticalbody within these constraints, at least some combination of second,third and fourth higher-order reflections can be suppressed without asubstantial decrease of the first harmonic reflection with angle ofincidence, particularly when the first reflection band is in theinfrared region of the spectrum. Films and optical bodies made inaccordance with the present invention are therefore particularly usefulas IR mirrors, and may be used advantageously as window films and insimilar applications where IR protection is desired but goodtransparency and low color are important.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in detail by way of reference to thefollowing drawings, without, however, the intention to limit theinvention thereto:

FIG. 1 is a schematic illustration of an optical repeating unit inaccordance with the present invention;

FIG. 2 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction arc related asspecified in Case 1 of Table I;

FIG. 3 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction are related asspecified in Case 2 of Table I;

FIG. 4 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction are related asspecified in Case 3 of Table I;

FIG. 5 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction are related asspecified in Case 4 of Table I;

FIG. 6 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction are related asspecified in Case 5 of Table I;

FIG. 7 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction are related asspecified in Case 6 of Table I;

FIG. 8 is a graph of reflectance as a function of angle of incidence forthe repeat unit of FIG. 1 when the indices of refraction are isotropic;and

FIG. 9 is a graph of measured transmittance as a function of wavelengthfor a sample in which the repeat unit has the index of refractionrelationship specified in Case 1 of Table I.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions and conventions are used throughout thedisclosure:

Multilayer film: a film comprising an optical repeating unit designed toreflect light over a particular range of wavelengths. The multilayerfilm may contain additional layers between the optical repeating unitswhich may or may not be repeated throughout the multilayer film.

Optical repeating unit: a stack of layers arranged in a particulararrangement which is repeated across the thickness of a multilayer film.

In-plane axes: two mutually perpendicular axes disposed in the plane ofthe film. In the present application, these axes will typically bedesignated as the x-axis and the y-axis.

Transverse axis: an axis that is perpendicular to the plane of the film.In the present application, this axis will typically be designated asthe z-axis.

The index of refraction for light polarized along a particular axis willtypically be denoted as n_(i), wherein i indicates the particular axis(i.e., n_(x) denotes the index of refraction for light polarized alongthe x axis). The normalized index difference is the difference betweenrefractive indices divided by the average of those indices. Thisaccounts for dispersion (i.e., changes in refractive index withwavelength).

Negative birefringence: the situation in which the index of refractionalong the transverse axis is less than the index of refraction along oneor both in-plane axes (n_(z)<n_(x) and/or n_(y))

Positive birefringence: the situation in which the index of refractionalong the transverse axis is greater than the index of refraction alongone or both in-plane axes (n_(z)>n_(x) and/or n_(y))

Isotropic: the situation in which the indices of refraction along the x,y and z-axes are the same (i.e., n_(x)=n_(y)=n_(z))

Infrared region: 700 nm to 2500 nm

Visible region: 400 nm to 700 nm

Optical thickness: the ratio defined as:$f^{k} = \frac{n^{k}*d^{k}}{\sum\limits_{M = 1}^{l}{n^{M}d^{M}}}$

wherein f^(k) is the optical thickness of polymeric layer k, 1 is thenumber of layers in the optical repeating unit, n^(k) is the refractiveindex of polymeric layer k, and d^(k) is the thickness of polymericlayer k. N^(M) is the refractive index of the M^(th) polymeric layer,and d^(M) is the thickness of the M^(th) polymeric layer. The opticalthickness ratio of polymeric layer k along an optical axis j is denotedas f_(j) ^(k) and is defined as above but with replacement of n^(k) withthe refractive index of polymeric material k along axis j (n_(j) ^(k)).

Skin layer: a layer that is provided as an outermost layer. Typically,skin layers in the films and optical bodies of the present inventionwill have a thickness between 10% and 20% of the sum of the physicalthicknesses of all optical repeating units.

Monotonically varying thickness of an optical repeating unit along amultilayer film: the situation in which the thickness of the opticalrepeating unit either consistently decreases or consistently increasesacross the thickness of the film (e.g., the thickness of the opticalrepeating unit does not show an increasing trend along part of thethickness of the film and a decreasing trend along another part of thethickness of the film).

In accordance with one embodiment of this invention, a film or otheroptical body is provided which has an optical repeating unit comprisinga multilayer stack containing m layers, where m is an integer of 4 ormore. Such an optical repeating unit includes polymeric layers A, B andC, which are preferably arranged in an optical repeating unit having thelayer sequence ABCB. The optical thickness ratio for each of thepolymeric layers preferably have the values f_(x) ^(a)=⅓, f_(x) ^(b)=⅙and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y) ^(c)=⅓,wherein the second, thrid, and forth reflection harmonics aresuppressed.

A schematic drawing of such a repeating unit is shown in FIG. 1.However, the values for the optical thickness ratios are not a necessarycondition of this embodiment, as other optical thickness ratios can alsoresult in suppression of some combination of second, third andforth-order reflection harmonics.

In the films and other optical bodies made in accordance with thepresent invention, it is preferred that at least one of the differencein indices of refraction between layers A and B along the z-axis (n_(z)^(a)−n_(z) ^(b)) and the difference in indices of refraction betweenlayers B and C along the z-axis (n_(z) ^(b)−n_(z) ^(c)) is negative.More preferably, at least one of these normalized differences is lessthan or equal to −0.03, and most preferably, at least one of thesenormalized differences is less than or equal to −0.06. In a particularlypreferred embodiment of the present invention, the optical repeatingunit is designed such that at least one of these normalized differencesis less than 0 and preferably is less than or equal to −0.03, and suchthat the other difference is less than or equal to 0. Most preferably,both differences are less than 0. It has been found that such designsyield the most robust performance and exhibit an increase in thereflection of p-polarized light (and therefore, in total reflection)with increasing angle of incidence.

It is also possible to design a film or other optical body in accordancewith the present invention which has an optical repeating unit in whichboth differences are substantially 0, i.e., wherein the absolute valueof the normalized differences is preferably less than about 0.03. Whenboth differences are substantially 0, there will be little or nodecrease of the infrared reflection of p-polarized light with the angleof incidence.

According to a still further embodiment of the present invention, one ofthe differences in refractive index between layers A and B across thez-axis is of opposite sign to the difference in refractive index betweenlayers B and C across the z-axis. In this embodiment, it is preferredthat either the difference that is less than 0 has the largest absolutevalue or that the absolute value of both differences is substantiallyequal. Films and other optical bodies in accordance with this embodimentwill have a substantially constant or increasing reflectance forp-polarized light with increasing angle of incidence, yielding asubstantially increasing reflectance for unpolarized incident light withincreasing incidence angle.

While the above described embodiments all yield optical repeating unitswhich substantially suppress some combination of the second, third andfourth higher-order reflection harmonics, and for which there is eitheran increase of the p-polarized infrared reflection with an increase inangle of incidence of the light or a substantially constant p-polarizedinfrared reflection when the angle of incidence increases, it has beenfound that, when both differences are substantially larger than 0 orwhen one of them is substantially larger than 0 and the other isessentially 0, a substantial and unacceptable decrease of infraredreflection of p-polarized light is noticed as angle of incidenceincreases, leading to a decrease in reflectance for randomly polarizedincident light. An example of this is illustrated in FIG. 8.

The behavior of the infrared reflection with angle of incidence isdepicted for each of the above described embodiments for opticalthickness ratios f_(x) ^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/orf_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y) ^(b)=⅓, in FIGS. 2-7. FIG. 2 showsthe infrared reflection for p-polarized and s-polarized light with angleof incidence for an optical repeating unit wherein the difference n_(z)^(a)−n_(z) ^(b) is −0.14 and the difference n_(z) ^(b)−n_(z) ^(c) is+0.13 and in which the optical repeating unit is otherwise designed suchthat the in-plane index relationships are in accordance with the presentinvention. In FIG. 3, the difference n_(z) ^(a)−n_(z) ^(b) is +0.15, andthe difference n_(z) ^(b)−n_(z) ^(c) is −0.17. The behavior of both thep-polarized light and s-polarized light are shown since both componentsof light determine the total amount of light reflected for the typicalinstance of unpolarized incident light. However, the present inventionallows for control (increasing reflectance with increasing angle ofincidence) of the p-polarized component of the reflected light in anovel and unexpected way.

The present invention is further illustrated by reference to thecomparative example shown in FIG. 8 with the examples of FIGS. 2 or 3.FIG. 8 illustrates how the reflectance changes with angle of incidencefor repeat units in which the transverse index relationships amongcomponents A, B and C are not controlled in accordance with the presentinvention. FIG. 8 illustrates the typical variation of the amount ofreflection of p-polarized and s-polarized light from the repeat stack,with the angle of incidence of light, where the difference n_(z)^(a)−n_(z) ^(b) is −0.15 and the difference n_(z) ^(b)−n_(z) ^(c) is−0.13 . The materials A, B and C are each isotropic; their refractiveindices are the same along all three axes. The overall reflectance forunpolarized incident light (the average of P and S-polarizedreflectance) substantially decreases with increasing incidence angle. Asseen from FIG. 8, there is a substantial decrease of the reflection ofp-polarized light when the angle of incidence increases. In the exampleshown in FIG. 8, the components of the repeating unit A, B and C, areisotropic, meaning that the in-plane and transverse indices are equal.By contrast, a further embodiment in accordance with the presentinvention for which both differences are very small compared to the inplane index differences, exhibits substantially constant reflection ofinfrared p-polarized light as the angle of incidence is varied (FIG. 4).FIG. 4 shows a typical variation of the amount of reflection ofp-polarized and s-polarized light from the repeat stack, where thedifference n_(z) ^(a)−n_(z) ^(b) is 0 and the difference n_(z)^(b)−n_(z) ^(c) is 0 (Table 1 Case 3).

FIG. 5 shows an embodiment wherein n_(z) ^(a)−n_(z) ^(b) is −0.13 andwherein the difference n_(z) ^(b)−n_(z) ^(c) is −0.15 . (Table 1 Case4). As can be seen from this figure, the infrared reflection forp-polarized light in this embodiment strongly increases with anincreasing angle of incidence. Similarly, FIG. 6 shows the reflectancebehavior for embodiments wherein n_(z) ^(a)−n_(z) ^(b) is −0.13 and thedifference n_(z) ^(b)−n_(z) ^(c) is 0 (Table 1 Case 5), and FIG. 7 showsthe reflectance behavior for embodiments wherein n_(z) ^(a)−n_(z) ^(b)is 0 and the difference n_(z) ^(b)−n_(z) ^(c) is −0.13. As can be seenfrom these figures, the infrared reflection of p-polarized light in thesystems of FIGS. 6 and 7, as with the system of FIG. 5, increases withangle of incidence.

FIG. 9 illustrates the measured spectra of a film made in accordancewith the present invention. The film stack uses a 4 layer repeat unit ofthe type ABCB wherein A is PEN, B is coPEN and C is PMMA. The stack iscomposed of a total of 15 repeat units. The overall reflectance of theaverage of S-polarized and P-polarized light, increases with incidenceangle. The refractive indices for polymers A,B and C used in thisexample are substantially identified by those in Case 1 Table 1 shownbelow. In this example polymeric layers A, B and C have refractive indexvalues such that n_(x) ^(b)=(n_(x) ^(a)n_(x) ^(c))^(1/2) and/or n_(y)^(b)=(n_(y) ^(a)n_(y) ^(c))^(1/2) (square-root condition) while keepingthe following in-plane optical thickness ratios: f_(x) ^(a)=⅓, f_(x)^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓ f_(y) ^(b)=⅙ and f_(y)^(c)=⅓. The P-polarized reflectance at 60 degrees is substantially thesame (with a change in wavelength location) as it is for normallyincident light. As a result, the overall reflectance for unpolarizedincident light (the average of P and S-polarized reflectance)substantially increases with increasing incidence angle.

In a further aspect of the present invention, the polymeric layers A, Band C have, along at least one in-plane axis, refractive indices whichdiffer from each other. In particular, the refractive indices are suchthat the refractive index of polymeric layer B is intermediate to thatof layers A and C along at least one in-plane axis. Furthermore, sincepolymeric layer A has the highest refractive index along at least onein-plane axis, the indices of refraction are in accordance with at leastone of the relations specified in Formulas II and III:

n _(x) ^(a) >n _(x) ^(b) >n _(x) ^(c)  (Formula II)

n _(y) ^(a) >n _(y) ^(b) >n _(y) ^(c)  (Formula III)

In the case where only one of Formulas II and III are fulfilled (e.g.,where n^(a)>n^(b)>n^(c) along only one in-plane axis), the relationshipalong the other in-plane axis may be of any kind; preferably, however,the indices of refraction are substantially equal along this axis. Filmsand other optical bodies made in accordance with this embodiment willsubstantially reflect light polarized along the first in-plane axis andwill substantially transmit light polarized along the other in-planeaxis, leading to a reflective polarizer in the wavelength rangeencompassed by the first harmonic reflection.

In an especially preferred embodiment of the present invention, theoptical repeat unit is designed such that the refractive indexrelationship in accordance with this invention is fulfilled along bothin-plane axes, thereby yielding an optical repeating unit capable ofreflecting light independent of its plane of polarization. IR mirrorsmade in accordance with this embodiment reflect infrared radiation butare substantially transparent to visible radiation, i.e., higher orderreflections in the visible region of the spectrum are suppressed.

By adjusting the optical thickness ratios along the particular in-planeaxis that has the index of refraction for polymeric layer B intermediatethat of polymeric layer A and polymeric layer C, at least two higherorder reflections for infrared light having its plane of polarizationparallel to that particular in-plane axis can be suppressed. It is,however, preferred that the index of refraction for polymeric layer B beintermediate that of polymeric layers A and C along both in-plane axesand, by adjusting the optical thickness ratios along both in-plane axes,an infrared reflective mirror can be obtained for which at least twosuccessive higher order reflections are suppressed. Such an infraredreflective mirror will be substantially transparent in the visibleregion and will be free of color (e.g., iridescence).

In another especially preferred embodiment of the present invention, therefractive indices for polymeric layers A, B and C are such that n_(x)^(b)=(n_(x) ^(a)n_(x) ^(c))^(1/2) and/or n_(y) ^(b)=(n_(y) ^(a)n_(y)^(c))^(1/2) (square-root condition), and layers A, B, and C have thefollowing in-plane optical thickness ratios: f_(x) ^(a)=⅓, f_(x) ^(b)=⅙and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y) ^(c)=⅓.Such a configuration will consist of a repeat unit consisting ofmaterial A with optical thickness f_(x) ^(a)=⅓, followed by material Bwith f_(x) ^(b)=⅙, followed by material C with f_(x) ^(c)=⅓, finallyfollowed by material B again with f_(x) ^(b)=⅙. Such a repeat cell issymbolically represented as ABCB. Embodiments of the present inventionwhich have this repeat cell are capable of suppressing second, third andfourth order reflections for normally incident light. Accordingly, areflective film designed according to this embodiment can be used toreflect infrared light up to about 2000 nm without introducingreflections in the visible region of the spectrum.

The particular refractive index relationships in accordance with thepresent invention can be obtained by appropriate selection of thepolymeric materials used for each of the individual layers. The presentinvention typically requires that at least one of the polymeric layersA, B and C is a birefringent polymer. One or more of the other layersmay be birefringent as well, or the other layers may be isotropic.Depending upon the particular polymer or polymer blend selected for apolymeric layer and on the processing conditions used to produce theoptical repeating unit, a polymeric layer can be negativelybirefringent, positively birefringent, or isotropic. The following table(Table I) shows embodiments that can yield optical repeating units inaccordance with the present invention (in particular, the especiallypreferred embodiment described above, or other embodiments wherein f_(x)^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙and f_(y) ^(c)=⅓, but for which the square-root condition is notsatisfied) leading to increasing levels of reflectance with increasingangles of incidence.

The relationships among the refractive indices along the z-axis describethe invention for the more general case in which the in-plane index ofrefraction of polymer B is intermediate to that of polymers A and C, andthe in-plane index of refraction of polymer A is greater than that ofpolymer C, with at least two successive higher-order harmonics beingsuppressed. It is to be understood that Table I shows generalrelationships among refractive indices for a given set of in-planerefractive indices, and that the amount of the difference in the indexof refraction along the z-axis among the polymeric layers A, B and C inaccordance with this invention will depend on the amount ofbirefringence of the birefringent layer(s).

TABLE I Example Cases (ABCB Config.) in which Reflectivity Increaseswith Incidence Angle Ny and/or Sign (N_(y) ^(a) − Sign(N_(z) ^(a) −Sign(N_(z) ^(b) − Case Nx Nz Type N_(y) ^(b)) Sign (N_(y) ^(b) − N_(y)^(c)) N_(z) ^(b)) N_(z) ^(b)) 1 |N_(z) ^(a) − N_(z) ^(b)| ≧ |N_(z) ^(b)− N_(z) ^(c)| A 1.78 1.49 B(+) (+) (−) B 1.63 1.63 I (+) (+) C 1.50 1.50I 2 |N_(z) ^(a) − N_(z) ^(b)| ≦ |N_(z) ^(b) − N_(z) ^(c)| A 1.78 1.78 I(+) (+) B 1.63 1.63 I (+) (−) C 1.50 1.80 B(−) 3 A 1.78 1.50 B(+) (+) 0B 1.63 1.50 B(+) (+) 0 C 1.50 1.50 I 4 A 1.78 1.50 B(+) (+) (−) B 1.631.63 I (+) (−) C 1.50 1.78 B(−) 5 A 1.78 1.50 B(+) (+) (−) B 1.63 1.63 I(+) 0 C 1.50 1.63 B(−) 6 A 1.78 1.63 B(+) (+) 0 B 1.63 1.63 I (+) (−) C1.50 1.76 B(−) Note: B(+) => negatively birefringent material B(−) =>positively birefringent material I => Isotropic material

The physical thickness of the individual polymeric layers A, B and C isgenerally selected so as to obtain a desired optical thickness ratio asexplained above. Accordingly, the particular physical thickness of alayer is not a primary concern (of course, the physical thickness partlydefines the optical thickness and the optical thickness of the opticalrepeat unit determines the wavelengths of the reflected light). However,the physical thickness of polymeric layers A, B and C is typically lessthan about 0.5 micrometers.

It is further preferred in the films and optical devices of the presentinvention that the normalized refractive indices between the polymers A,B and C are at least about 0.03 along an in-plane axis for which therefractive index relationship is in accordance with Formula II orFormula III. Thus, it is preferred that the normalized differencesbetween n_(x) ^(a), n_(x) ^(b) and n_(x) ^(c) are at least about 0.03and/or that the normalized differences between n_(y) ^(a), n_(y) ^(b)and n_(y) ^(c) differ from each other by at least about 0.03.

The especially preferred embodiment described above in which polymericlayers A, B and C have refractive indices such that n_(x) ^(b)=(n_(x)^(a)n_(x) ^(c))^(1/2) and/or n_(y) ^(b)=(n_(y) ^(a)n_(y) ^(c))^(1/2),and having the in-plane optical thickness ratios f_(x) ^(a)=⅓, f_(x)^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y)^(c)=⅓, will suppress the second, third and forth reflection harmonicsfor normally incident light. When the incident light is non-normallyincident, these higher-order reflection harmonics may becomeunsuppressed to a degree, depending on the polarization of the incidentlight, and the refractive index relationships among the in-plane and thez-axis of each polymeric material. Indeed, the degree to whichhigher-order harmonics, which are suppressed at normal incidence,provide reflectance at higher incidence angles can be substantial,leading to films which become colored or which are reflective to onepolarization state at high incidence angles. Such optical properties canbe controlled by specifying the index relationships described in Table Iabove. Each case will have increasing reflection of the first-orderharmonic with increasing incidence angle, but will have differingamounts of increase in reflectance for the higher-order reflectionharmonics (from zero at normal angle) with increasing angle ofincidence. For example, Case 3 of Table I will result in negligibleincrease in reflectance with increasing angle (reflectance remainsnearly zero), while Case 4 of Table I will exhibit a significantincrease in reflectance for the higher-order harmonics with increasingangle of incidence.

Similarly, refractive index dispersion (changes in the in-plane axis andtransverse axis refractive indices, with wavelength) can result in adegree of non-suppression of higher-order harmonics in certainwavelength region, even though the conditions for complete higher-orderharmonic suppression are met in other wavelength regions. The degree towhich refractive index dispersion can change the degree of higher-orderharmonic suppression depends on the dispersion characteristic of theparticular polymeric material comprising the repeat cell; certainpolymers have greater dispersion than others. Such effects can beameliorated by choices of polymers A, B and C, and by techniques such asdesigning the f-ratios to “best match” the required values forhigher-order harmonic suppression across the wavelength range ofinterest. Indeed, both the effects of index dispersion and angle ofincidence (described above) on causing a degree of non-suppression ofhigher-order harmonics can be minimized through polymer choice and“best-match” f-ratio design. The best match f-ratio design may include avariation in f-ratio distribution as a function of total repeat unitthickness. The distribution of repeat unit thickness may likewise beoptimized.

Embodiments of the present invention for which the polymeric materialsA, B and C have in-plane optical thickness ratios f_(x) ^(a)=⅓, f_(x)^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y) ^(b)=⅙ and f_(y)^(c)=⅓, but which do not satisfy the condition n_(x) ^(b)=(n_(x)^(a)n_(x) ^(c))^(1/2) and or n_(y) ^(b)=(n_(y) ^(a) ^(c))^(1/2) (thesquare-root condition), will not have simultaneous suppression of thesecond, third and fourth-order reflection harmonics for normallyincident light. When the in-plane refractive indices deviate from thesquare-root condition, some combination of the second, third and/orforth-order reflection harmonics will develop reflectance while theother(s) will remain suppressed. The details of this departure fromsuppression depend on the optical thickness ratios of the polymers A, Band C, and on the way in which the square-root condition is violated.

Other embodiments of the present invention which satisfy Formulas IIand/or III, and which have a unit cell arrangement ABCB, can havein-plane refractive indices that do not satisfy the square rootcondition, and also do not have the in-plane optical thickness ratiosf_(x) ^(a)=⅓, f_(x) ^(b)=⅙ and f_(x) ^(c)=⅓ and/or f_(y) ^(a)=⅓, f_(y)^(b)=⅙ and f_(y) ^(c)=⅓. In such instances, various combinations ofsecond, third and forth-order reflection harmonics can be suppressed,and control of the first-order reflection can be maintained as describedabove. Table II below shows several examples of this.

TABLE II Example Cases (ABCB Config.) in which Reflectivity Increaseswith Incidence Angle Ny and/or Sign(N_(y) ^(a) − Sign(N_(y) ^(b) −Sign(N_(z) ^(a) − Sign(N_(z) ^(b) − Case Nx Nz Type N_(y) ^(b)) N_(y)^(c)) N_(z) ^(b)) N_(z) ^(c)) 7 A 1.78 1.50 B(+) (+) 0 B 1.67 1.50 B(+)(+) 0 C 1.50 1.50 I 8 A 1.78 1.50 B(+) (+) (−) B 1.67 1.67 I (+) (−) C1.50 1.78 B(−) 9 A 1.78 1.50 B(+) (+) (−) B 1.67 1.67 I (+) 0 C 1.501.63 B(−) 10 A 1.78 1.63 B(+) (+) 0 B 1.67 1.67 I (+) (−) C 1.50 1.76B(−) Note: B(+) => negatively birefringent material B(−) => positivelybirefringent material I => Isotropic material

The optical thickness ratios for polymeric materials A, B and C may havevalues that allow for suppression of differing combinations of thesecond, third and/or fourth-order reflection harmonics. Determiningrequired values of refractive indicies and f-ratios for polymers A, Band C, for suppression of combinations of two or more higher-orderharmonics, for isotropic materials at normal incidence, is explained inthe art. Such values may be determined by analytic techniques when boththe refractive index values and the f-ratios are considered unknowns(c.f. Much more R, B., J. Opt. Soc. Am., 38 20,(1948), and Thelen, A.,J.Opt.Soc. Am. 53, 1266 (1963) ), or through numerical techniques whenthe refractive index values are fixed by realistic polymer choices.

For example, for certain polymer refractive index values of n_(x)^(b)=1.0255(n_(x) ^(a)n_(x) ^(c))^(1/2) and/or n_(y) ^(b)=1.0255(n_(y)^(a) _(y) ^(c))^(1/2) and n_(y) ^(a)=1.772, n_(y) ^(c)=1.497 and/orn_(x) ^(a)=1.772 and n_(x) ^(c)=1.497 if f_(x) ^(a)=0.200, f_(x)^(b)=0.200 and f_(x) ^(c)=0.400 and/or f_(y) ^(a)=0.200, f_(y)^(b)=0.200 and f_(y) ^(c)=0.400, then the second and third-orderreflection harmonics will be suppressed. If, however, f_(x)^(a)=(0.3846), f_(x) ^(b)=(0.1538) and f_(x) ^(c)=(0.3077) and/or f_(y)^(a)=(0.3846), f_(y) ^(b)=(0.1538) and f_(y) ^(c)=(0.3077), then onlythe third and the forth-order reflection harmonics will be suppressed.As discussed above, higher-order harmonic suppression will occur fornormally incident light, and suppression (or lack thereof) of thehigher-order harmonics for non-normally incident light, or in wavelengthregions with high refractive index dispersion, will differ for each caseillustrated in Table II.

Cases 3 through 6 in Table I, and Cases 7 through 10 in Table II,illustrate specific examples of a more general result: Any polymericrepeating unit arranged in a multilayer stack consisting of polymericmaterials P₁, P₂, P₃, . . . P_(m), wherein the sign of the difference ofin-plane refractive indices between adjacent polymers P_(i) and P_(i+1)is opposite the sign of the difference in the z-axis refractive indicesbetween the same P_(i) and P_(i+1) for all polymeric interfaces, orwherein the sign of the difference in the z-axis refractive indicesbetween the same P_(i) and P_(i+1) is equal to 0 for all polymerinterfaces, will have increasing reflectance (of the first-orderharmonic) of unpolarized incident light with increasing angle ofincidence.

In the films and other optical bodies produced in accordance with thepresent invention which are designed as IR reflectors, it is generallypreferred that the polymeric layers of the optical repeating unit showsubstantially no absorption in the visible part of the spectrum unlesssome color tint is desired. An infrared reflective film produced inaccordance with the present invention preferably reflects infrared lightover a wide range of wavelengths, and accordingly an optical thicknessvariation is preferably introduced for the optical repeating unit alongthe thickness of the reflective film. In certain embodiments, sequencesof optical repeat units with monotonically increasing and decreasingoptical thickness are desired. Methods for designing optical thicknessgradients for the optical repeat units are set forth in U.S. Pat. No.6,157,490 entitled “Optical Film with Sharpened Bandedge”, andincorporated herein by reference. The optical thickness of the opticalrepeating unit may monotonically increase or decrease along the infraredreflective film. Typically, an infrared reflective film in connectionwith the present invention can be designed to have an infraredreflective bandwidth of 200 nm to 1000 nm for a given optical repeatunit.

One skilled in the art will appreciate that a wide variety of materialscan be used to form mirrors or polarizers according to the presentinvention when these materials are processed under conditions selectedto yield the desired refractive index relationships. The desiredrefractive index relationships can be achieved in a variety of ways,including stretching during or after film formation (e.g., in the caseof organic polymers), extrusion (e.g., in the case of liquid crystallinematerials), or coating. It is preferred, however, that the two materialshave similar rheological properties (e.g., melt viscosities) so thatthey can be co-extruded.

In general, appropriate combinations may be achieved by selecting foreach of layers A, B and C, a crystalline, semi-crystalline, or liquidcrystalline material, or an amorphous polymer. Of course, it is to beunderstood that, in the polymer art, it is generally recognized thatpolymers are typically not entirely crystalline, and therefore in thecontext of the present invention, crystalline orsemi-crystalline-polymers refer to those polymers that are not amorphousand includes any of those materials commonly referred to as crystalline,partially crystalline, or semi-crystalline.

Specific examples of suitable materials include polyethylene naphthalate(PEN) and isomers thereof(e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN),polyalkylene terephthalates (e.g., polyethylene terephthalate (PET),polybutylene terephthalate (PBT),poly-1,4-cyclohexanedimethyleneterephthalate) and copolymers of these,e.g., PETG, polyimides (e.g., polyacrylic imides), polyetherimides,polycarbonates (including copolymers such as the copolycarbonate of4,4′-thiodiphenol and bisphenol A in a 3:1 molar ratio, i.e., TDP),polymethacrylates(e.g., polyisobutyl methacrylate,polypropylmethacrylate,polyethylmethacrylate,andpolymethylmethacrylate),polyacrylates(e.g., polybutylacrylate andpolymethylacrylate),atactic polystyrene, syndiotactic polystyrene (sPS),syndiotactic poly-alpha-methyl styrene, syndiotacticpolydichlorostyrene, copolymers and blends of any of these polystyrenes,cellulose derivatives (e.g., ethyl cellulose, cellulose acetate,cellulose propionate, cellulose acetate butyrate, and cellulosenitrate), polyalkylene polymers (e.g., polyethylene, polypropylene,polybutylene, polyisobutylene, and poly(4-methyl)pentene),fluorinatedpolymers (e.g., perfluoroalkoxy resins,polytetrafluoroethylene,fluorinated ethylene-propylene copolymers,polyvinylidene fluoride, and polychlorotrifluoroethylene),chlorinatedpolymers (e.g., polyvinylidenechloride andpolyvinylchloride),polysulfones,polyethersulfones,polyacrylonitrile,polyamides, silicone resins, epoxyresins, polyvinylacetate,polyether-amides, ionomeric resins, elastomers(e.g.,. polybutadiene, polyisoprene, and neoprene), and polyurethanes.Also suitable are various copolymers, e.g., copolymers of PEN (e.g.,copolymers of 2,6-, 1,4-, 1,5-, 2,7-, and/or 2,3-naphthalenedicarboxylic acid, or esters thereof, with (a) terephthalic acid, oresters thereof; (b) isophthalic acid, or esters thereof; (c) phthalicacid, or esters thereof; (d) alkane glycols; (e) cycloalkane glycols(e.g., cyclohexane dimethanol diol); (f) alkane dicarboxylic acids;and/or (g) cycloalkane dicarboxylic acids (e.g., cyclohexanedicarboxylic acid)), copolymers of polyalkylene terephthalates (e.g.,copolymers of terephthalic acid, or esters thereof, with (a) naphthalenedicarboxylic acid, or esters thereof; (b) isophthalic acid, or estersthereof; (c) phthalic acid, or esters thereof; (d) alkane glycols; (e)cycloalkane glycols (e.g., cyclohexane dimethane diol); (f) alkanedicarboxylic acids; and/or (g) cycloalkane dicarboxylic acids (e.g.,cyclohexane dicarboxylic acid)), and styrene copolymers (e.g.,styrene-butadiene copolymers and styrene-acrylonitrile copolymers),4,4′-bibenzoic acid and ethylene glycol. In addition, each individuallayer may include blends of two or more of the above-described polymersor copolymers (e.g., blends of syndiotactic polystyrene (sPS) andatactic polystyrene).

A particularly preferred birefringent polymeric material for use in oneor more layers of the films and other optical devices produced inaccordance with the present invention is a crystalline orsemi-crystalline polyethylenenaphthalate (PEN), inclusive of its isomers(e.g. 2,6-; 1,4-; 1,5-; 2,7; and 2,3-PEN). Particularly preferredisotropic polymeric materials for use in the present invention includepolyacrylates and in particular, polymethylmethacrylate (PMMA). Oneskilled in the art will appreciate that each of the polymeric layers A,B and C may be composed of blends of two or more polymeric materials toobtain desired properties for a specific layer.

Preferred optical repeating units in connection with the presentinvention are optical repeating units that have polymeric layers A, Band C arranged in an order AABCCB. Especially preferred are thosewherein layers A, B and C comprise the various sets of polymers asdescribed in the following paragraphs.

The following material embodiments for the various optical cases arenon-exclusive examples based on modeled data, since many such systemsexist. For example, PBN (poly butylene naphthalate) or even anon-polyester can be used for the material of highest in-plane index.Index of refraction values are approximate and assume a wavelength of632.8 nm. Copolymers of PEN with other polyesters are listed ascoPEN×/100−x, where x is the approximate percentage of NDC (naphthalenedicarboxylate) content and can vary by more than +/−20%. Unlessotherwise noted, the coPENs are considered essentially unoriented byattention to the process conditions and by suitable selection of thecomponents in the non-NDC fraction (100−x).

The example material systems listed in TABLE III below refer to the caseexamples illustrated in TABLE I and TABLE II and FIGS. 2-7. Some of theexample material cases involve very similar polymers, but achievedifferent refractive index results through differing process conditions.

TABLE III Case No.* Materials Approx N_(y) and N_(x) Approx N_(z) Case 1A = PEN 1.74 1.48 B = coPEN 60/40 1.61 1.61 C = PMMA 1.49 1.49 - OR - A= PEN 1.74 1.48 B = coPEN 70/30 1.62 1.62 C = ECDEL ™ 1.52 1.52 (hereECDEL ™ is one of many aliphatic polyesters with this low value ofisotropic index of refraction) - OR - A = PEN 1.74 1.48 B = TDP 1.631.63 C = ECDEL ™ 1.52 1.52 - OR - A = PET 1.65 1.49 B = PC 1.57 1.57 C =PMMA 1.49 1.49 (Here PC can be a standard bis-phenol A type.) - OR - A =PET 1.65 1.49 B = PETG 1.57 1.57 C = PMMA 1.49 1.49 Case 7 A = PEN 1.741.48 B = PET 1.65 1.49 C = PMMA 1.49 1.49 Case 3 A = PEN 1.74 1.48 B =PBT 1.63 1.47 C = PMMA 1.49 1.49 Case 4 A = PET 1.65 1.49 B = coPEN50/50 1.60 1.60 C = sPS 1.55 1.63 Case 5 A = PEN 1.74 1.50 B = coPEN1.63 1.63 C = sPS 1.55 1.63 *(see TABLES I, II)

This invention also anticipates the formation of a multicomponentpolarizing film for longer wavelengths (e.g., the near IR) which wouldalso be substantially transparent in the visible region of the spectrum.In TABLES I and II, this is the case of one in-plane index set meetingthe required conditions while the orthogonal set of in-plane indices aresubstantially matched across the IR spectrum of interest. Examples ofsuch films may be constructed using the multiple drawing step processesin U.S. Ser. No. 09/006,455. One particular combination for materials Aand C may include PEN and a copolymer comprising 10% of PEN typesubunits and 90% of PET type subunits, i.e., an orienting andcrystallizable 10/90 co-PEN (as is obtained from a coextrudedtransesterified blend of 10 weight % PEN and 90 weight % PET). Thechoice of material B could be an intermediate copolymer of these, e.g.,an orienting and crystallizable 70/30 co-PEN. In general, a variety ofIR polarizers could be constructed by a variety of material combinationswith this method.

The films and other optical devices made in accordance with theinvention may also include one or more anti-reflective layers orcoatings, such as, for example, conventional vacuum coated dielectricmetal oxide or metal/metal oxide optical films, silica sol gel coatings,and coated or coextruded antireflective layers such as those derivedfrom low index fluoropolymers such as THV, an extrudable fluoropolymeravailable from 3M Company (St. Paul, Minn.). Such layers or coatings,which may or may not be polarization sensitive, serve to increasetransmission and to reduce reflective glare, and may be imparted to thefilms and optical devices of the present invention through appropriatesurface treatment, such as coating or sputter etching.

Both visible and near IR dyes and pigments are contemplated for use inthe films and other optical bodies of the present invention, andinclude, for example, optical brighteners such as dyes that absorb inthe UV and fluoresce in the visible region of the color spectrum. Otheradditional layers that may be added to alter the appearance of theoptical film include, for example, opacifying (black) layers, diffusinglayers, holographic images or holographic diffusers, and metal layers.Each of these may be applied directly to one or both surfaces of theoptical film, or may be a component of a second film or foilconstruction that is laminated to the optical film. Alternately, somecomponents such as opacifying or diffusing agents, or colored pigments,may be included in an adhesive layer which is used to laminate theoptical film to another surface.

Suitable methods for making reflective multilayer films of the typetoward which the present invention is directed are described, forexample, in U.S. Ser. No. 09/006,288 which is hereby incorporated byreference. However, some of the considerations involved in these methodsare discussed briefly below.

It is preferred that the polymers have compatible rheologies tofacilitate coextrusion. That is, since the use of coextrusion techniquesis preferred in making the films and other optical bodies of the presentinvention, the melt viscosities of the polymers are preferablyreasonably matched to prevent layer instability or non-uniformity. Thepolymers used also preferably have sufficient interfacial adhesion sothat the resulting films will not delaminate.

The multilayer reflective films of the present invention can be readilymanufactured in a cost effective way, and can be formed and shaped intoa variety of useful configurations after coextrusion. Multilayerinfrared reflective films in accordance with the present invention aremost advantageously prepared by employing multilayered coextrusiondevices such as those described in U.S. Pat. Nos. 3,773,882 and3,884,606, the disclosures of which are incorporated herein byreference. Such devices provide a method for preparing multilayered,simultaneously extruded thermoplastic materials, each of which are of asubstantially uniform layer thickness. Preferably, a series of layermultiplying means as are described in U.S. Pat. No. 3,759,647, thedisclosure of which is incorporated herein by reference, are employed.

The feedblock of the coextrusion device receives streams of the diversethermoplastic polymeric materials from a source such as a heatplastifying extruder. The streams of resinous materials are passed to amechanical manipulating section within the feedblock. This sectionserves to rearrange the original streams into a multilayered streamhaving the number of layers desired in the final body. Optionally, thismultilayered stream may be subsequently passed through a series of layermultiplying means in order to further increase the number of layers inthe final body.

The multilayered stream is then passed into an extrusion die which is soconstructed and arranged that stream-lined flow is maintained therein.Such extrusion devices are described in U.S. Pat. No. 3,557,265, thedisclosure of which is incorporated by reference. The resultant productis extruded to form a multilayered body in which each layer is generallyparallel to the major surface of adjacent layers.

The configuration of the extrusion die may vary and can be such as toreduce the thickness and dimensions of each of the layers. The precisedegree of reduction in thickness of the layers delivered from themechanical orienting section, the configuration of the die, and theamount of mechanical working of the body after extrusion are all factorswhich affect the thickness of the individual layers in the final body.

The number of layers in the reflective films and other optical devicesmade in accordance with the present invention can be selected to achievethe desired optical properties using the minimum number of layers forreasons of film thickness, flexibility and economy. In the case of bothinfrared reflective polarizers and infrared reflective mirrors, thenumber of layers is preferably less than about 10,000, more preferablyless than about 5,000, and most preferably, less than about 2,000.

The desired relationship between refractive indices of polymeric layersA, B and C as desired in this invention can be achieved by selection ofappropriate processing conditions. In the case of organic polymers whichcan be oriented by stretching, the multilayer films are generallyprepared by co-extruding the individual polymers to form a multilayerfilm (e.g., as set out above) and then orienting the film by stretchingat a selected temperature, optionally followed by heat-setting at aselected temperature. Alternatively, the extrusion and orientation stepsmay be performed simultaneously. By the orientation, the desired extentof birefringence (negative or positive) is set in those polymeric layersthat comprise a polymer that can exhibit a birefringence. Positivebirefringence is obtained with polymers that show a negative opticalstress coefficient, i.e., polymers for which the in-plane indices willdecrease with orientation whereas negative birefringence is obtainedwith polymers having a positive optical stress coefficient.

In the case of polarizers, the film is typically stretched substantiallyin one direction (uniaxial orientation), while in the case of mirrors,the film can be stretched substantially in two directions (biaxialorientation). However, with the proper selection of conditions,polarizing films can be made through biaxial orientation. Such films maybe made, for example, by stretching the film under such conditions thatparticular layers are selectively oriented and other layers are not.Suitable methods for producing biaxial polarizers in accordance with thepresent invention are described, for example, in U.S. Ser. No.09/006,455.

In the case of mirrors, the stretching may be asymmetric to introducespecially desired features, but is preferably symmetric. Reflectivemirrors may also be obtained in accordance with the present invention bylaminating together two infrared reflective films that have each beenuniaxially oriented in such a way that their axes of orientation arerotated 90° to one another.

The film may be allowed to dimensionally relax in the cross-stretchdirection from the natural reduction in cross-stretch (equal to thesquare root of the stretch ratio), or may be constrained so that thereis no substantial change in cross-stretch dimensions. The film may bestretched in the machine direction, as with a length orienter, and/or inthe transverse or width direction using a tenter.

The pre-stretch temperature, stretch temperature, stretch rate, stretchratio, heat set temperature, heat set time, heat set relaxation, andcross-stretch relaxation are selected to yield a multilayer devicehaving the desired refractive index relationship. These variables areinter-dependent; thus, for example, a relatively low stretch rate couldbe used if coupled, for example, with a relatively low stretchtemperature. One skilled in the art will appreciate that variouscombinations of these variables may be selected to achieve a desiredmultilayer device. In general, however, a stretch ratio in the rangefrom about 1:2 to about 1:10 (more preferably about 1:3 to about 1:7) inthe stretch direction and from about 1:0.2 to about 1:10 (morepreferably from about 1:0.2 to about 1:7) orthogonal to the stretchdirection is preferred.

Suitable multilayer films may also be prepared using techniques such asspin coating (e.g., as described in Boese et al., J. Polym. Sci.: PartB, 30:1321 (1992) for birefringent polyimides), and vacuum deposition(e.g., as described by Zang et. al., Appl. Phys. Letters, 59:823 (1991)for crystalline organic compounds). The latter technique is particularlyuseful for certain combinations of crystalline organic compounds andinorganic materials.

Orientation of the extruded film can be accomplished by stretchingindividual sheets of the material in heated air. For economicalproduction, stretching may be accomplished on a continuous basis in astandard length orienter, tenter oven, or both. Economies of scale andline speeds of standard polymer film production may be achieved, therebyreducing manufacturing costs below levels associated with commerciallyavailable absorptive polarizers.

Lamination of two or more reflective films together is advantageous toimprove reflectivity or to broaden the bandwidth, or to form a mirrorfrom two polarizers as described above. Amorphous copolyesters, suchVITEL™ 3000 and 3300 which are commercially available from the GoodyearTire and Rubber Co. of Akron, Ohio, are useful as laminating materials.The choice of laminating material is broad, with degree of adhesion,optical clarity and exclusion of air being the primary guidingprinciples.

It may be desirable to add to one or more of the layers one or moreinorganic or organic adjuvants such as an antioxidant, extrusion aid,heat stabilizer, ultraviolet ray absorber, nucleator, surface projectionforming agent, and the like in normal quantities so long as the additiondoes not substantially interfere with the performance of the presentinvention.

A practical situation that can arise in the selection of a glue or “tie”layer is that it is common for elastomers, polyolefins, and otherpolymers which are good candidate tie layer materials to be isotropicand have the lowest refractive index of any of the repeat cellcomponents (often they are in the 1.47-1.52 range). Issues of materialcost, optical haze, absorptive color, or overall mechanical propertiesmay preclude using the thickness of the tie layer required for a twocomponent system consisting of the low index tie layer with a high indexmaterial in a 2 component repeat cell. A solution to this problem is todesign the 3 component repeat cell in a pattern A/C/B/C wheren^(a)>n^(b)>n^(c) in at least one in-plane direction and wherein C isthe aforementioned tie layer having the lowest index (it is alsopossible that Nb=Nc). For the aforementioned reasons it may be desirableto make the thickness of the C layer as small as possible to minimizethe cost and optical haze/absorptive color problems.

If a film of the present invention is designed to reflect light in theinfrared region, it may be preferable to further design the film toavoid a change in perceived color as the viewing angle or angle ofincidence of light changes, for example, from normal incidence tonon-normal incidence, while maintaining the ability to provide infraredblocking properties over as much of the infrared region of the spectrumas possible. For typical dielectric multilayer films, if the reflectingband is positioned to reflect the maximum amount of solar radiation atnormal angles of incidence while remaining clear in the visible regionof the spectrum, the short wavelength bandedge is positioned at or nearthe visible wavelength cutoff, i.e. at about 700 nm. The reflecting bandmoves to shorter wavelengths at non-normal angles of incidence, however,so that while the film appears clear at normal angles, it will becolored at non-normal angles.

For some applications, it is desirable that the film appear clear at allangles of light incidence or viewing angles, and to accomplish this, thereflecting band must be positioned at longer wavelengths within theinfrared so that the short wavelength bandedge does not shift into thevisible region of the spectrum even at maximum use angles. This can beaccomplished by designing an infrared reflecting film of the presentinvention so that the film has a reflecting band positioned to reflectinfrared radiation of at least one polarization at an incident anglenormal to the film, where the reflecting band has a short wavelengthbandedge λ_(a0) and long wavelength bandedge λ_(b0) at a normal incidentangle, and a short wavelength bandedge λ_(aθ)and long wavelengthbandedge λ_(bθ)at a maximum usage angle θ, wherein λ_(aθ)is less thanλ_(a0) and λ_(a0) is selectively positioned at a wavelength greater thanabout 700 nm. At least one component can then be provided as part of thefilm or in addition to the film which at least partially absorbs orreflects radiation in the wavelength region between λ_(aθ)and λ_(a0) ata normal angle of incidence.

The component functions to either absorb or reflect the infraredwavelengths that are not reflected by the film at normal angles becauseof the positioning of the reflective band of the multilayer film athigher wavelengths in order to minimize perceived color changes atnon-normal incidence. Depending on the placement of the gap fillercomponent relative to the film, the component may not function atnon-normal angles because the reflective band gap due to the reflectionof the multilayer film shifts to lower wavelengths, preferablycoinciding with the wavelength region of the absorption or reflection ofthe gap filler component. Placement of the reflective band within theinfrared and components useful for filling the resulting band gap thatoccurs at normal angles are more fully described in U.S. Pat. No.6,049,419, incorporated herein by reference.

Suitable gap filler components include an infrared absorbing dye orpigment, an infrared absorbing glass, a trailing segment, a plurality ofisotropic layers, or combinations thereof. The gap filler component maybe a part of the film, for example, as a trailing segment or a pluralityof isotropic layers coextruded with the film layers or as a dye orpigment incorporated into one or more of the film layers.

Alternatively, the gap filler component may be a discrete part of theoptical body of the present invention, i.e., separate from the film,that is attached, for example, laminated thereto. Examples of thisembodiment include a dye or pigment as a separate layer adhered to thefilm. The description of the gap filler as a part of the film andseparate from the film is merely exemplary. The gap filler componentdisclosed herein may be either be a part of the film or may be separatefrom the film depending on the characteristics of the component itselfand the film with which it is being combined.

The film and the gap filler components are preferably combined such thatthe film is placed on a surface nearest the sun as practical because itis more efficient to reflect solar energy than to absorb it. In otherwords, where possible, it is preferable that the sun's rays firstencounter the film and then secondarily encounter the gap fillercomponent. In a multiple pane or two-ply windshield, the most preferableplacement for the film is the exterior nearest the sun, the nextpreferably position is between the panes or plies. The film may beplaced on the interior surface but this allows absorption of solar lightby the glass before the light reaches the film and absorption of part ofthe light reflected from the film. This embodiment may be preferablewhen considered from a UV protection standpoint, since it may bepreferable to position the film away from the sun, allowing componentswhich are less sensitive to UV to absorb this part of the light.

Examples of suitable infrared absorbing dyes include cyanine dyes asdescribed, for example, in U.S. Pat. No. 4,973,572, hereby incorporatedby reference, as well as bridged cyanine dyes and trinuclear cyaninedyes as described, for example, in U.S. Pat. No. 5,034,303, herebyincorporated by reference, merocyanine dyes as described, for example,in U.S. Pat. No. 4,950,640, hereby incorporated by reference,carbocyanine dyes (for example, 3,3′-diethyloxatricarbocyanine iodide,1,1′, 3,3,3′, 3′-hexamethylindotricarbocyanine perchlorate,1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide,3,3′-diethylthiatricarbocyanine iodide, 3,3′-diethylthiatricarbocyanineperchlorate, 1,1′,3,3,3′, 3′-hexamethyl-4,4′,5,5′-dibenzo-2,2′-indotricarbocyanine perchlorate, all of which arecommercially available from Kodak, Rochester, N.Y.), and phthalocyaninedyes as described, for example, in U.S. Pat. No. 4,788,128, herebyincorporated by reference; naphthaline dyes; metal complex dyes, forexample, metal dithiolate dyes (for example, nickel dithiolate dyes and,for example, bis[4-dimethylaminodithiobenzil] nickel, bis[dithiobenzil]nickel, bis[1,2-bis(n-butylthio)ethene-1,2-dithiol]nickel bis[4,4′-dimethoxydithiobenzil] nickel, bis[dithiobenzil] platinum,bis[dithioacetyl] nickel) and metal dithiolene dyes (for example,nickeldithiolene dyes as described, for example, in U.S. Pat. No.5,036,040, hereby incorporated by reference); polymethine dyes such asbis(chalcogenopyrylo)polymethine dyes as described, for example, in U.S.Pat. No. 4,948,777, hereby incorporated by reference, bis(aminoaryl)polymethine dyes as described, for example, in U.S. Pat. No. 4,950,639,hereby incorporated by reference, indene-bridged polymethine dyes asdescribed, for example, in U.S. Pat. No. 5,019,480, hereby incorporatedby reference, and tetraaryl polymethine dyes; diphenylmethane dyes;triphenylmethane dyes; quinone dyes; azo dyes; ferrous complexes asdescribed, for example, in U.S. Pat. No. 4,912,083, hereby incorporatedby reference; squarylium dyes as described, for example, in U.S. Pat.No. 4,942,141, hereby incorporated by reference;chalcogenopyryloarylidene dyes as described, for example, in U.S. Pat.No. 4,948,776, hereby incorporated by reference; oxoindolizine dyes asdescribed, for example, in U.S. Pat. No. 4,948,778, hereby incorporatedby reference; anthraquinone and naphthoquinone derived dyes asdescribed, for example, in U.S. Pat. No. 4,952,552, hereby incorporatedby reference; pyrrocoline dyes as described, for example, in U.S. Pat.No. 5,196,393, hereby incorporated by reference; oxonol dyes asdescribed, for example, in U.S. Pat. No. 5,035,977, hereby incorporatedby reference; squaraine dyes such as chromylium squaraine dyes,thiopyrylium squaraine dyes as described, for example, in U.S. Pat. No.5,019,549, hereby incorporated by reference, and thiochromyliumsquaraine dyes; polyisothianaphthene dyes; indoaniline and azomethinedyes as described, for example, in U.S. Pat. No. 5,193,737, herebyincorporated by reference; indoaniline methide dyes; tetraarylaminiumradical cation dyes and metallized quinoline indoaniline dyes.Squarylium dyes or squaraines are also described, for example, in U.S.Pat. No. 4,942,141 and U.S. Pat. No. 5,019,549, both of which are herebyincorporated by reference.

Commercially available phthalocyanine dyes include, for example, thoseavailable from Zeneca Corporation, Blackley, Manchester, England underthe trade designation “Projet Series” for example, “Projet 830NP”,“Projet 860 NP” and “Projet 900NP”.

Commercially available metal complex dyes include those available fromC. C. Scientific Products, Ft. Worth, Tex. 76120, for example,bis[4-dimethylaminodithiobenzil] nickel.

Additional suitable dyes include those described in Jurgen Fabian'sarticle entitled “Near Infrared Absorbing Dyes” Chem Rev, 1992,1197-1226 and “The Sigma Aldrich Handbook of Stains, Dyes andIndicators” by Floyd J. Green, Aldrich Chemical Company, Inc.,Milwaukee, Wis. ISBN 0-941633-22-5,1991, both of which are herebyincorporated by reference. Useful near infrared absorbing dyes includethose from Epolin, Inc., Newark, N.J., for example, having the tradedesignations: Epolight III-57, Epolight III-117, Epolight V-79, EpolightV-138, Epolight V-129, Epolight V-99, Epolight V-130, Epolight V-149,Epolight IV-66, Epolight IV-62A, and Epolight III-189.

Suitable infrared absorbing pigments include cyanines, metal oxides andsquaraines. Suitable pigments include those described in U.S. Pat. No.5,215,838, incorporated herein by reference, such as metalphthalocyanines, for example, vanadyl phthalocyanine, chloroindiumphthalocyanine, titanyl phthalocyanine, chloroaluminum phthalocyanine,copper phthalocyanine, magnesium phthalocyanine, and the like;squaraines, such as hydroxy squaraine, and the like; as well as mixturesthereof. Exemplary copper pthalocyanine pigments include the pigmentcommercially available from BASF under the trade designation “6912”.Other exemplary infrared pigments include the metal oxide pigmentcommercially available from Heubach Langelsheim under the tradedesignation “Heucodor”.

Dyes or pigments useful in the present invention may be narrow-bandabsorbing, absorbing in the region of the spectrum not covered becauseof the position of the short wavelength bandedge of the optical body,for example, 700 to 850 nm, or may be broad band, absorbing oversubstantially all or all of the infrared region.

The dye or pigment can be applied to either surface of the film, in alayer of glass or polymer, such as polycarbonate or acrylic, laminatedto the film, or be present in at least one of the polymer layers of thefilm. From a solar energy standpoint, the dye is preferably on theinnermost surface of the film (i.e. toward the room interior and awayfrom the sun) so that when the sun is a high angle, the film reflectiveband shifts to lower wavelengths, essentially coinciding with thewavelength region of the dye. This is preferred because reflecting solarenergy away from the building is preferred to absorbing it.

The amount of dye or pigment used in the optical body of the presentinvention varies depending on the type of dye or pigment and/or the enduse application. Typically, when applied to the surface of the film, thedye or pigment is present on the surface at a concentration and coatingthickness suitable to accomplish the desired infrared absorption andvisible appearance. Typically, if the dye or pigment is within anadditional layer or within the multilayer optical body, theconcentration ranges from about 0.05 to about 0.5 weight %, based on thetotal weight of the optical body. In addition, when a pigment is used, asmall particle size typically is needed, for example, less than thewavelength of light. If the dyes are non-polar solvent soluble, the dyescan be coated or mixed in with solid plastic pellets and extruded if thedyes can withstand the heat of mixing and extrusion.

Examples of suitable infrared absorbing glasses include clear glasshaving a thickness generally ranging from about 3 to about 6 mm, such asarchitectural or automotive glass; blue glass; or green glass whichselectively absorb in the near infrared, i.e., about 700 to 1800 nm.

In the embodiments where blue or green glass is used, it is preferablethat the film of the present invention is located on the surface of theglass closest to the sun so that the film can reflect away the 850-1250nm wavelengths, allowing some of the infrared which is not reflected tobe absorbed by the glass. If it is not practical to place the film onthe exterior surface of a glass layer, for example, on the exterior of awindow of a building, it may be useful to place the film between panesof glass, rather than on the surface closest to the interior, in thecase of multiple pane windows, in order to minimize absorption.Preferably, the exterior layer (closest to the sun) has minimal infraredabsorbing properties so that the film is able to reflect light in theinfrared region before this light reaches the interior infraredabsorbing glass. In this embodiment, the glass temperature would belower and less heat would enter the room due to re-radiation of absorbedlight. Additionally, the glass and/or film would be cooler which wouldreduce cracking of the glass due to thermal stress, a common problemwith heavily absorbing materials.

Infrared absorbing glass is available commercially from companiesincluding Pittsburgh Plate Glass (PPG), Guardian, Pilkington-LibbeyOwens Ford, Toledo, Ohio.

Generally a sharp band edge is desired in optical interference filmssuch as the infrared reflective films described herein. Sharp band edgescan be obtained from proper design of the layer thickness gradientthroughout the multilayer optical stack, as described in U.S. Pat. No.6,157,490. Instead, a reflective film of the present invention can bedesigned to include a trailing segment to partially reflect infraredwavelengths in the gap region without producing strong color in thevisible spectrum at non-normal angles. A trailing segment can beprovided as a multilayer interference film have layer thicknesses andrefractive indices such that the reflectance in the gap region isrelatively weak, for example, 50% and which may decrease so thattransfer from high reflectance to low reflectance of the multilayer filmis gradual. For example, a layer gradient may provide a sharp bandedgeabove, for example, the 50% reflectance point and a trailing segmentcould be provided by additional layers. For example, instead ofproviding a sharp edge, the last 30 layers of a 200 layer stack could beof appropriate optical thickness that their first order reflectionoccurs in the range of about 800-850 nm, the intensity of which increasefrom about 90% reflection at 850 nm to about 25% at 800 nm. The other170 layers could provide, for example, about 90% reflection from about850-1150 nm. Achieving the trailing segment can be done in a number ofways, for example, by controlling the volumetric feed of the individuallayers. The trailing segment may be extruded with the multilayer film ofthe present invention or laminated thereto.

Possible advantages of a trailing segment is that instead of an abrupttransition from no color to maximum color, the trailing segment providesa “softer” transition which may be more aesthetically acceptable andeasier to control from a process standpoint.

An isotropic multilayer film may also be used to cover at least aportion of the wavelength gap. Isotropic layers lose p-pol reflectionintensity at oblique angles. Accordingly, at oblique angles, the z-indexmatched reflectance band would shift into the gap and the reflectancefrom the isotropic layers would shift to the visible but also decreasein p-pol intensity. S-pol would be masked or partially masked by theair/optical body surface which would increase its reflectance at obliqueangles. Exemplary isotropic polymers include but are not limited toisotropic coPEN, PMMA, polycarbonates, styrene acrylonitriles, PETG,PCTG, styrenics, polyurethanes,polyolefins, and fluoropolymers. Theisotropic film may be coextruded with a film of the present invention orlaminated to a film of the present invention.

Preferably, gap filler component is situated such that light hits thefilm of the present invention before it hits the gap filler component sothat, then when the sun is at normal incidence, the gap filler absorbslight in the region of the gap. However, when the sun is at high angles,the film will shift to some of the same wavelengths as the gap fillercomponent and serve to reflect at least some of the light in the regionof the gap.

Gap filler components may be used in combination with the film of thepresent invention, for example, when each gap filler component onlyabsorbs or reflects in a portion of the gap to be filled. In addition,shifting the bandedge and, thus, creating the gap, also serves to createanother, or second, gap in the infrared region at longer wavelengths offangle. Therefore, it may be preferable to also include another componentwhich fills this second gap region off angle. Suitable gap fillercomponents to fill this second gap include dyes, pigments, glasses,metals and multilayer films which absorb or reflect in the longerwavelengths of the infrared region.

The preceding description of the present invention is merelyillustrative, and is not intended to be limiting. Therefore, the scopeof the present invention should be construed solely by reference to theappended claims.

What is claimed is:
 1. An infrared reflecting mirror comprising a firstreflective film that reflects light in the infrared region of thespectrum while transmitting light in the visible region of the spectrum,wherein the first reflective film comprises a multilayer film comprisingan optical repeating unit that comprises polymeric layers A, B and Carranged in an order ABC, the polymeric layer A having refractiveindices n_(x) ^(a) and n_(y) ^(a) along in-plane axes x and yrespectively, the polymeric layer B having refractive indices n_(x) ^(b)and n_(y) ^(b) along in-plane axes x and y respectively, the polymericlayer C having refractive indices n_(x) ^(c) and n_(y) ^(c) alongin-plane axes x and y respectively, polymeric layers A, B and C having arefractive index n_(z) ^(a), n_(z) ^(b) and n_(z) ^(c) respectivelyalong a transverse axis z perpendicular to the in-plane axes, whereinn_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c), or n_(y) ^(a)>n_(y) ^(b)>n_(y) ^(c),or both n_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c) and n_(y) ^(a)>n_(y) ^(b)>n_(y)_(c), and wherein at least one of the differences n_(z) ^(a)−n_(z) ^(b)or n_(z) ^(b)−n_(z) ^(c) is less than 0 or both the differences aresubstantially equal to
 0. 2. An infrared reflecting mirror according toclaim 1 further comprising a second reflective film that reflects lightin the infrared region of the spectrum while transmitting light in thevisible region of the spectrum, wherein the first and second reflectivefilms are uniaxially oriented and arranged such that their axes oforientation are rotated 90° to one another, and wherein the secondreflective film comprises a multilayer film comprising an opticalrepeating unit that comprises polymeric layers A, B and C arranged in anorder ABC, the polymeric layer A having refractive indices n_(x) ^(a)and n_(y) ^(a) along in-plane axes x and y respectively, the polymericlayer B having refractive indices n_(x) ^(b) and n_(y) ^(b) alongin-plane axes x and y respectively, the polymeric layer C havingrefractive indices n_(x) ^(c) and n_(y) ^(c) along in-plane axes x and yrespectively, polymeric layers A, B and C having a refractive indexn_(z) ^(a), n_(z) ^(b) and n_(z) ^(c) respectively along a transverseaxis z perpendicular to the in-plane axes, wherein n_(x) ^(a)>n_(x)^(b)>n_(x) ^(c), or n_(y) ^(a)>n_(y) ^(b)>n_(y) ^(c), or both n_(x)^(a)>n_(x) ^(b)>n_(x) ^(c) and n_(y) ^(a)>n_(y) ^(b)>n_(y) ^(c), andwherein at least one of the differences n_(z) ^(a)−n_(z) ^(b) or n_(z)^(b) is less than 0 or both the differences are substantially equal to0.
 3. A method of reflecting infrared light comprising the steps ofproviding an infrared reflecting mirror according to claim 1 andallowing at least a portion of incident infrared light to reflect fromsaid reflective film.
 4. The method of claim 3, wherein at least 50percent of incident infrared light having a wavelength of between 750 nmand 1600 nm is reflected.
 5. A material comprising on a support areflective film that reflects light in the infrared region of thespectrum while transmitting light in the visible region of the spectrum,wherein the reflective film comprises a multilayer film comprising anoptical repeating unit that comprises polymeric layers A, B and Carranged in an order ABC, the polymeric layer A having refractiveindices n_(x) ^(a) and n_(y) ^(a) along in-plane axes x and yrespectively, the polymeric layer B having refractive indices n_(x) ^(b)and n_(y) ^(b) along in-plane axes x and y respectively, the polymericlayer C having refractive indices n_(x) ^(c) and n_(y) ^(c) alongin-plane axes x and y respectively, polymeric layers A, B and C having arefractive index n_(z) ^(a), n_(z) ^(b) and n_(z) ^(c) respectivelyalong a transverse axis z perpendicular to the in-plane axes, whereinn_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c), or n_(y) ^(a)>n_(y) ^(b)>n_(y) ^(c),or both n_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c) and n_(y) ^(a)>n_(y) ^(b)>n_(y)^(c), and wherein at least one of the differences n_(z) ^(a)−n_(z) ^(b)or n_(z) ^(b)−n_(z) ^(c) is less than 0 or both the differences aresubstantially equal to
 0. 6. The material of claim 5, wherein thesupport is transparent to visible light.
 7. The material claim 5,wherein the support is glass or a plastic film.
 8. A method ofreflecting infrared light comprising the steps of providing a reflectivefilm that reflects light in the infrared region of the spectrum whiletransmitting light in the visible region of the spectrum and allowing atleast a portion of incident infrared light to reflect from thereflective film, wherein the reflective film comprises a multilayer filmcomprising an optical repeating unit that comprises polymeric layers A,B and C arranged in an order ABC, the polymeric layer A havingrefractive indices n_(x) ^(a) and n_(y) ^(a) along in-plane axes x and yrespectively, the polymeric layer B having refractive indices n_(x) ^(b)and n_(y) ^(b) along in-plane axes x and y respectively, the polymericlayer C having refractive indices n_(x) ^(c) and n_(y) ^(c) alongin-plane axes x and y respectively, polymeric layers A, B and C having arefractive index n_(z) ^(a), n_(z) ^(b) and n_(z) ^(c) respectivelyalong a transverse axis z perpendicular to the in-plane axes, whereinn_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c), or n_(y) ^(a)>n_(y) ^(b)>n_(y) ^(c),or both n_(x) ^(a)>n_(x) ^(b)>n_(x) ^(c) and n_(y) ^(a)>n_(y) ^(b)>n_(y)^(c), and wherein at least one of the differences n_(z) ^(a)−n_(z) ^(b)or n_(z) ^(b)−n_(z) ^(c) is less than 0 or both the differences aresubstantially equal to
 0. 9. The method of claim 8, wherein at least 50percent of incident infrared light having a wavelength between 750 nmand 1600 nm is reflected.
 10. An optical body comprising: a firstpolymeric layer A; a second polymeric layer B; and a third polymericlayer C; wherein A, B, and C, are arranged in the sequence ABC, where Bis contiguous to A and C; wherein A has refractive indices n_(x) ^(A),n_(y) ^(A), and n_(z) ^(A) for light polarized along mutually orthogonalaxes x, y, and z, respectively, B has refractive indices n_(x) ^(B),n_(y) ^(B), and n_(z) ^(B) for light polarized along axes x, y, and z,respectively, and C has refractive indices n_(x) ^(C), n_(y) ^(C), andn_(z) ^(C) for light polarized along axes x, y, and z, respectively;wherein axis z is orthogonal to layer B; and wherein the indices ofrefraction of layers A, B, and C are in accordance with Formula I and atleast one of Formulas II and III: n _(z) ^(C) ≧n _(z) ^(B) ≧n _(z)^(A)  (Formula I) n _(x) ^(A) >n _(x) ^(B) >n _(x) ^(C)  (Formula II) n_(y) ^(A) >n _(y) ^(B) >n _(y) ^(C)  (Formula III).
 11. The optical bodyof claim 10, wherein the optical body comprises a plurality of repeatingunits having the layer sequence ABC.
 12. The optical body of claim 10,wherein the optical body comprises a plurality of repeating units havingthe layer sequence ABCB.
 13. The optical body of claim 11, wherein A, B,and C have optical thickness ratios f_(x) ^(a), f_(x) ^(b), and f_(x)^(c), respectively, along in-plane axis x, and f_(y) ^(a), f_(y) ^(b),and f_(y) ^(c), respectively, along in-plane axis y, and wherein f_(x)^(a)=⅓, f_(x) ^(b)=⅙, f_(x) ^(c)=⅓, and n_(x) ^(b)=(n_(x) ^(a)n_(x)^(c))^(1/2) or wherein f_(y) ^(a)=⅓, f_(y) ^(b)=⅙, f_(y) ^(c)=⅓, andn_(y) ^(b) =(n_(y) ^(a)n_(y) ^(c))^(1/2).
 14. The optical body of claim11, wherein A, B, and C have optical thickness ratios f_(x) ^(a), f_(x)^(b), and f_(x) ^(c), respectively, along in-plane axis x, and f_(y)^(a), f_(y) ^(b), and f_(y) ^(c), respectively, along in-plane axis y,and wherein f_(x) ^(a)=⅓, f_(x) ^(b)=⅙, f_(x) ^(c)=⅓, and n_(x)^(b)=(n_(x) ^(a)n_(x) ^(c))^(1/2) and wherein f_(y) ^(a)=⅓, f_(y)^(b)=⅙, f_(y) ^(c)=⅓, and n_(y) ^(b)=(n_(y) ^(a)n_(y) ^(c))^(1/2).